There are three major methods of optically transferring a photomask pattern to a wafer. These are contact printing, proximity printing, and projection printing. In the fabrication of integrated circuits projection printing is used almost exclusively. In most conventional systems, each photomask is fabricated to include the primary pattern (the boundaries of the over-all circuit). Once a mask is complete, it is printed many times on a wafer surface using a step-and-repeat process which is well-known in the art. The result is a pattern of dies covering the wafer surface.
In order to efficiently use the available wafer surface, the ability to accurately and precisely transfer patterns onto the wafer surface is critical. Thus, the equipment used to project patterns onto the wafer surface must provide, among other aspects, good resolution. The term `resolution` describes the ability of the optical system to distinguish closely-spaced objects. The resolution of the optical lithography printing system is of major importance, since it is the main limitation of minimum device size. In modern projection printers the quality of the optical elements is so high that their imaging characteristics are limited by diffraction effects, and not by lens aberrations (diffraction limited systems).
There are a variety of photomasks used in conventional very large scale integration (VLSI) circuit fabrication processes. One widely used type is the attenuated phase-shifting mask, which has the advantage of being a two layer structure, simplifying the manufacturing process. Attenuated phase-shifting masks, described in Burn J. Lin, "The Attenuated Phase-Shift Mask", 43-47 Solid State Technology (January 1992), use a slightly transmissive absorber with a 180.degree. phase-shift in the place of the opaque material in the mask pattern. Unlike many other phase-shifting masks, attenuated phase-shifting masks can be used for any arbitrary mask pattern. An attenuated phase-shifting mask shifts the phase of dark areas but with an attenuated amplitude to prevent producing too much light in those areas. The negative amplitude provides the desired improvement in image edge contrast, while the attenuation prevents the negative amplitude from becoming a problem by controlling the intensity. Resolution of closely packed features is further improved when using an attenuated phase-shifting mask incorporating a mask bias, because exposure times and diffractive effects can be reduced.
When a primary mask pattern is fabricated using attenuating phase shifting material, the determination of where to locate adjacent dies must provide for at least two considerations. One is that the mask plate cannot extend to the edge of the die because leakage through the attenuated material at the edge of the primary pattern produces a shading effect, reducing the resolution of the pattern edge. The other concern is that, if adjacent dies are located too close together, the leakage at the edge of the mask pattern when printing one die will detrimentally effect the neighboring die(s). In order to make efficient use of the available wafer surface, there is a need to minimize the detrimental optical effects occurring at the edges of the primary mask pattern. This would allow more precise definition of die boundaries and closer placement of adjacent dies.
It is known in the art to employ diffraction gratings to improve mask feature resolution by turning optical effects such as diffraction to an advantage. Conventional diffraction gratings consist of rectangular features arranged at equidistant intervals. It should be noted that, in integrated circuit fabrication, a diffraction grating is not printed per se. Instead, the grating is employed to control optical effects such as diffraction. The pattern of the grating produces destructive interference, thereby providing some amount of control over intensity patterns on the wafer surface.
One example of a use for such patterns, often referred to as "subresolution gratings" or "zero electrical field gratings", is in the manufacture of attenuated phase-shifting masks, where the gratings are employed to reduce the amount of light going through the attenuated material at the edge of the primary pattern. The subresolution gratings used in conventional processing control optical effects by manipulating both the exposure parameters and the size and relative placement of rectangular contacts. FIG. 1 shows an example of a portion of a mask incorporating a conventional subresolution grating. In the example shown, contacts 110 are separated by a space equal to the dimension of the contacts 110.
FIG. 2 is a graphic representation showing subresolution grating leakage versus contact size for the conventional subresolution grating of FIG. 1. As shown in FIG. 2, the efficiency of the conventional grating is somewhat improved when the contact size is reduced for all numerical apertures evaluated. As can be seen from graph 200, however, the conventional subresolution grating pattern experiences a minimum residual intensity of at least 25% for even the smallest contact sizes. As a result, there is a point at which, using conventional methods, pattern resolution cannot be further improved. This creates a barrier to further efficiencies in printing mask patterns on a wafer. What is needed is a method for fabricating a photomask with reduced energy leakage at the edges of the primary pattern.